1. Field of the Invention
The present invention relates to a large-aperture telephoto lens system, and in particular, relates to an internal focusing large-aperture telephoto lens system which is suitable for use in an electronic still camera, in which a miniature solid-state image sensor is installed.
2. Description of Related Art
In the related art, an internal focusing telephoto lens system has been proposed with the objective of facilitating the focusing process and reducing the burden on the autofocus drive mechanism. In an internal focusing telephoto lens system, compared to a totally-telescoping lens system, it is possible to reduce movement distance of a focusing lens group that moves in the optical axis direction during a focusing operation and to reduce the weight of the focusing lens group. For example, in Japanese Unexamined Patent Publication No. H08-122629 (Patent Literature 1), Japanese Unexamined Patent Publication No. H09-159911 (Patent Literature 2), Japanese Unexamined Patent Publication No. 2002-107616 (Patent Literature 3), Japanese Unexamined Patent Publication No. 2008-145584 (Patent Literature 4) and Japanese Unexamined Patent Publication No. 2009-186609 (Patent Literature 5), internal focusing telephoto lens systems are disclosed which are configured of a positive first lens group, a negative second lens group and a positive third lens group, in that order from the object side, in which the second lens group is moved in the optical axis direction upon a focusing on an object at infinity through to an object at a finite distance.
On the other hand, miniature electronic still cameras, in which an image sensor that is sufficiently smaller than a conventional 35 mm or APS frame size, have been developed. In recent years, high specifications, especially in regard to a larger aperture diameter, have been in demand in the optical system used in such miniature electronic still cameras. Increasing the aperture diameter of an optical system is equivalent to increasing the effective aperture diameter of the lens groups, through which the axial light bundle passes. However, if the diameter and the weight of the focusing lens group increase, the burden on the autofocus drive mechanism increases. Furthermore, it also becomes difficult to attain a high imaging quality.
Such a telephoto lens system used in a miniature electronic still camera can be achieved by scaling down the optical system of a large-aperture telephoto lens system that is suitable for a convention large-sized image sensor such as those disclosed in Patent Literature 1 through 5. However, merely scaling down the optical system still results in the effective diameter of the focusing lens group being too big for use as a lens group for use in the above-described miniature camera. Whereas, if attempts are made to reduce the lens diameters in order to reduce the burden on the autofocus drive mechanism, it becomes difficult to achieve a high imaging quality.
In each of the telephoto lens systems disclosed in Patent Literature 1 through 5, since the optical-power balance is set inappropriately in each lens group of the optical system, the movement amount of the focusing lens group during focusing is large, the effective aperture diameter of the focusing lens group is large and the weight of the focusing lens group is large, thereby increasing the burden on the autofocus drive mechanism, and correction of various aberrations, such as spherical aberration, that occur during a focusing operation becomes difficult, so that the optical quality deteriorates.
Furthermore, in the internal focusing telephoto lens systems disclosed in Patent Literature 1 through 5, since the spherical aberration correction and coma correction in the first lens group, which has a strong positive refractive power, are inappropriate, a high imaging quality cannot be achieved and the aperture diameter cannot be suitably enlarged.